The invention relates to a hydraulic drive having a main pump, which can be controlled in terms of delivery volume and delivery direction, and having a hydraulic motor, which main pump and hydraulic motor are connected to one another in a closed circuit via pressure lines, and having a charge pump for charging into the closed circuit and for supplying pressurized fluid to at least one control device, as per the preamble of Claim 1.
A hydraulic drive of said type is known from DE 102 14 598 A1. Similar hydraulic drives are also described in documents DE 41 11 921 C2, DE 102 38 614 A1, DE 199 30 997 B4 and DE 100 37 195 A1. In such hydraulic drives, the charge pump has the function of providing pressurized fluid for the actuation of the control devices at least for the main pump and/or the variable displacement motor, and of compensating oil losses as a result of leakages in the closed circuit. The charge pump must be dimensioned for this purpose such that it maintains a certain low pressure level, usually in the range of 20-25 bar, under all circumstances. The charge pump is therefore designed such that said pressure level is obtained even at the lowest rotational speed of the drive. The power required for this can be up to 20% of that of the main pump, and is therefore considerable.
In said known drives, the charge pump is mechanically coupled to the main pump at all times and is driven together with the latter by the same internal combustion engine. The rotational speed of said drive is predefined by the rotational speed of the internal combustion engine, such that the delivery volume of the non-adjustable charge pump is dependent on, and fluctuates with, the engine rotational speed. Since the charge pump must provide a predefined minimum pressure in all rotational speed ranges, said charge pump must be of such large dimensions that it builds up a sufficient pressure even at a minimum rotational speed of the internal combustion engine. As a result, the charge pump is overdimensioned at relatively high rotational speed and delivers more pressurized fluid than is necessary. Said excess pressurized fluid is, according to the prior art, dissipated via pressure limiting valves and the energy contained therein is converted into waste heat. This leads to considerable power losses in the drive system, in particular at relatively high rotational speeds of the drive. Furthermore, coolants are required for dissipating the heat energy, which entails an increased material expenditure in the cooling system of the hydraulic machine.
In DE 102 38 614 A1, it is proposed, to avoid such losses, for the charge pump to be assisted in the low rotational speed range of the internal combustion engine by means of the provision of additional pressurized fluid by a further pump. Said further pump serves primarily for the supply to separate working hydraulics and is disconnected from the charge pressure circuit by means of a control valve at relatively high rotational speeds of the internal combustion engine. Although the charge pump itself can hereby be of smaller dimensions than previously conventional, said solution does however entail increased expenditure in terms of components and control measures.
The invention is therefore based on the object of creating a hydraulic drive of the type specified in the introduction in which the energy losses are considerably reduced in relation to the prior art and which can be realized with little expenditure.